The present invention relates generally to a method for making a condensing heat exchanger which can be used at higher temperatures by extrusion coating a melted thermoplastic polymer directly onto the heated surface of the metal condensing heat exchanger.
Condensing heat exchangers are employed in condensing furnaces to increase efficiency. The condensing heat exchanger cools the heating fluid to a temperature below the dew point. As the temperature drops below the dew point, a liquid condensate, water vapor, condenses from the heating fluid. As the liquid condensate condenses, heat is transferred from the water vapor to the air to be heated. As more heat is produced, the efficiency of the system is increased.
Polypropylene films are commonly utilized to make a laminated condensing heat exchanger material to prevent corrosion of the heat exchanger by the water vapor liquid condensate. In prior condensing heat exchangers, the film is formed by melted polypropylene pellets which are extruded or cast to form a film. The metal surface of the condensing heat exchanger is then heated. The film is applied to the heated condensing heat exchanger, and the heated surface slightly melts the film, adhering the film to the condensing heat exchanger. A roller passes over the surface of the film to further adhere the film to the condensing heat exchanger. Alternatively, the film is adhered to the metal with an appropriate adhesive and primer.
There are several drawbacks to the polypropylene film of the prior art. For one, the present polypropylene film can be used only at relatively lower temperatures. The prior art process can be used to make a film which can be used at higher temperatures, but manufacturing of these films is difficult and expensive as the surface of the heat exchanger must be heated to a relatively higher temperature in order to slightly melt and adhere the film to the condensing heat exchanger. Additionally, there is a limitation on how hot the metal heat exchanger can be heated in a commercial oven.
Hence, there is a need in the art for a method for making a condensing heat exchanger which can be used at higher temperatures by applying a film having high thermal and chemical resistance.
The present invention relates to a method for making a condensing heat exchanger which can be used at higher temperatures.
A thermoplastic polymer film is directly extruded onto the heated surface of a metal condensing heat exchanger. The thermoplastic polymer in the form of a plurality of pellets are dropped in a hollow cylinder and heated to form a melted thermoplastic polymer. An internal screw in the cylinder stirs and pushes the melted thermoplastic polymer through an orifice in the cylinder and directly onto the heated surface of the metal condensing heat exchanger. The melted thermoplastic polymer self-glues to the heated condensing heat exchanger, forming the thermoplastic film. A pressure roller is preferably utilized to bring the thermoplastic film into direct contact with the heated surface of the condensing heat exchanger.
Any thermoplastic polymer which is capable of being adhered to the heated metal surface of the condensing heat exchanger can be utilized to form the thermoplastic film. Preferably, the thermoplastic polymer is a polyester, such as polybutylene terephthalate or polyethylene terephthalate. Alternatively, a polyolefin, such as polypropylene or polyethylene, can be used as the thermoplastic polymer. A maleate or a tackifier must be mixed with a polyolefin for proper adhesion or bonding to the heated surface of the condensing heat exchanger. Polyetherimide, polyethersulfone, polysufone or polyimide can also be utilized as the thermoplastic polymer. Once adhered, the thermoplastic film must be resistant to the conditions of the condensing heat exchanger, such as high temperature flue vent gases and the acidic condensate formed during condensation.
Accordingly, the present invention provides a method for making a condensing heat exchanger which can be used at higher temperatures.
These and other features of the present invention will be best understood from the following specification and drawings.